Atomically Flat Dielectric Patterns for Band Gap Engineering and Lateral Junction Formation in MoSe$_2$ Monolayers

TL;DR

This study demonstrates a method to create atomically flat dielectric patterns on substrates, enabling the formation of lateral heterojunctions in MoSe₂ monolayers with tunable electronic and optical properties for advanced device applications.

Contribution

The paper introduces a precise sputter etching and ALD growth technique to fabricate flat dielectric patterns, enabling scalable lateral heterojunctions in MoSe₂ monolayers with tunable properties.

Findings

Significant CPD variation across dielectric interfaces.

Bandgap shifts observed via photoluminescence spectroscopy.

Differing carrier dynamics on SiO₂ and AlOₓ sides.

Abstract

Combining a precise sputter etching method with subsequent AlO$_x$ growth within an atomic layer deposition chamber enables fabrication of atomically flat lateral patterns of SiO$_2$ and AlO$_x$. The transfer of MoSe$_2$ monolayers onto these dielectrically modulated substrates results in formation of lateral heterojunctions, with the flat substrate topography leading to minimal strain across the junction. Kelvin probe force microscopy (KPFM) measurements show significant variations in the contact potential difference (CPD) across the interface, with AlO$_x$ regions inducing a 230~mV increase in CPD. Spatially resolved photoluminescence spectroscopy reveals shifts in spectral weight of neutral and charged exciton species across the different dielectric regions. On the AlO$_x$ side, the Fermi energy moves closer to the conduction band, leading to a higher trion-to-exciton ratio, indicating a bandgap shift consistent with CPD changes. In addition, transient reflection spectroscopy highlights the influence of the dielectric environment on carrier dynamics, with the SiO$_2$ side exhibiting rapid carrier decay typical of neutral exciton recombination. In contrast, the AlO$_x$ side shows slower, mixed decay behavior consistent with conversion of trions back into excitons. These results demonstrate how dielectric substrate engineering can tune the electronic and optical characteristics of proximal two-dimensional materials, allowing scalable fabrication of advanced junctions for novel (opto)electronics applications.